That was my first thought, yes, but then I think area ought to be involved somewhere; if the area is large enough, even a medium population will not have people

That was my first thought, yes, but then I think area ought to be involved somewhere; if the area is large enough, even a medium population will not have people ("particles") interacting ("colliding") as often. In the rather clumsy taxonomy of crime I created from the UCR, most violent crime -- excepting suicide -- would be collision-based. Some drug crimes like possession would not be collision-based (although it could be argued that possession involves buying which involves another person) while drug sales would be. Crimes against property are interesting -- is that another person by proxy, or should that merely be collision-less?